WASHINGTON--(BUSINESS WIRE)--Advanced military jet aircraft have engines that provide the needed
speed and maneuverability. However, with this greater power there is
significant noise during takeoff and landing. The noise can impact the
public and affect the health and safety of flight line workers. To
confront the challenges of the noise problem, the Naval Research
Laboratory (NRL) collaborated with the University of Cincinnati (UC) and
GE Aircraft Engines to investigate the use of mechanical chevrons
(serrations at the rim of the exhaust nozzle) and fluidics to
successfully reduce the noise from supersonic military jet aircraft.

The initial research, sponsored by the Strategic Environmental Research
and Development Program (SERDP), revealed the potential for significant
noise reduction of high performance jet engines without compromising
performance or environmental standards. SERDP is DoD’s environmental
science and technology program, planned and executed in partnership with
DOE and EPA. The Navy recognized this research as a Top Twenty Research
Accomplishment of 2009.

NRL scientists, Drs. Junhui Liu, Ravi Ramamurti and Kazhikathra
Kailasanath from the Laboratory for Computational Physics and Fluid
Dynamics, performed computational analysis to characterize the flow from
the nozzle exhaust of a supersonic jet engine. Their findings revealed
that the spacing of the shock cells and the length of the core increased
as the ratio of the pressure inside the nozzle to the pressure outside
(total pressure ratio) increased. These results were confirmed by
experimental data from UC, showing that computational analysis works in
assessing the effectiveness of chevrons and fluidics. The research team
published their finding in the American Institute of Aeronautics and
Astronautics journal and received a 2009 Alan Berman Research
Publication Award for outstanding archival research, as judged by NRL
management.

Following the successful simulations of the jet flow and noise, the
research team shifted their work to simulate and assess specific noise
reduction concepts. They conducted experiments and simulations to assess
the impact of the mechanical chevrons on the flow from the exhaust
nozzle and near-field noise. Their simulation results revealed that the
chevrons caused the shock cells to move closer to the nozzle and reduce
the spacing between them. The chevrons also induced more spread of the
jet flow and decreased the strength of the shock cells. Together, these
factors reduced the noise significantly, by more than 3 decibels at the
locations where they tested.

As a next step, the researchers replaced the mechanical chevrons with
fluidic injections of air at discrete locations along the jet nozzle
rim. Using a 1 to 2 percent injection of air under a variety of
operating conductions, the researchers achieved the same noise reduction
as they did using the mechanical chevrons. The advantage of the fluidic
injection technique is that it can be easily turned on and off.

The researchers then investigated a combination of mechanical chevrons
and fluidics. They were able to improve the effect of the chevrons on
flow modification and noise reduction by properly positioning the
fluidic injections of air. They also injected air upstream of the nozzle
exit, near the throat, to effectively modify the flow area. This
modified the shock cell structures and reduced the noise generated.

Working under the Office of Naval Research-Rapid Technology Transition
program, GE researchers then tested the mechanical chevrons on a
full-scale Navy engine. The tests, conducted at the NAWC-Lakehurst
Facility, proved the mechanical chevrons to be effective, as predicted
by the NRL simulations. Work will continue on this project, using the
validated computational capability, to further increase the noise
reduction level and advance the technology. ONR is funding this research.